To start off with, I'd stick to 5.1 and use the money saved on the second surround speakers to upgrade to QS8's, one of the larger centre channels and/or an EP500 sub or competitors equivalent. You can always upgrade to 7.1 at a later date if you feel like it without compromising on your main components up front.

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Half of communication is listening. You can't listen with your mouth.

If you follow the rule of thirds for standing waves, your sofa should be about 12' off the front wall. This puts your surrounds at 12.5-14 from front and only leaves 4-6 feet of breathing room for your rears. Not ideal.

Too small to take advantage of 7.1 imo.... rears will smear into surrounds and become indistinguishable. Soundfield collapses.